The present invention relates in general to variable resistors, and more specifically to a thick-film resistor card employed in a transducer for converting a physical position into an electrical signal.
Variable resistors are known for sensing parameters in various applications. For example, the fuel level in an automobile fuel tank is typically measured using a variable resistor having a sweep arm attached to a float. The sweep arm position is determined by the level of fuel in the tank and the voltage values sensed across the variable resistor depends on the position of the sweep arm.
The most successful prior art variable resistor design has been the stepped resistor. In one type of stepped resistor, a high resistance wire is wrapped around a core such that the sweep arm slides from one wrapped loop to the next, resulting in resistance steps. Another stepped design includes a silk screen printed thick-film resistor card in which resistor ink and conductor ink are deposited on a substrate. A large number of conductor lines are formed in parallel and extend from the area containing the deposited resistive material to form a sliding wiper contact area.
As the sweep-arm wiper contact moves from one step to the next, small circuit breaks occur. These breaks cause spikes in the electrical characteristic of the variable resistor. The spiking problem is particularly acute when using digital electronic gages which employ very low amperage signals (e.g., in the range of 10-20 milliamps). The spikes must be electrically filtered prior to applying the resistor output signal to the gage to avoid flickering of the digital display. Thus, it would be desirable to eliminate spiking and thereby avoid the need for a filter.
In order to provide the required accuracy in the desired resistance characteristic according to sweep arm angle, thick-film resistor cards must be calibrated on an individual basis. The desired resistance characteristic is determined so that resistance values along the sweep arm path correspond to volume values of fuel remaining in the fuel tank. Calibration is achieved by trimming resistance material in the cards. For accuracy, the resistance characteristic is monitored during the trimming adjustment. In stepped resistor designs, the resistance characteristic is monitored using measurement pads which are connected to selected ones of the conductor lines. Thus, while it is desirable to eliminate the steps in the resistor, it is desirable to maintain the ability to monitor the resistance characteristic.
It is also desirable that the variable resistor withstand wear so as to maintain the resistance characteristic over an extended lifetime. It is further desirable that the trimming and assembly operations of the variable resistor be adaptable to automation.
U.S. Pat. No. 4,318,075 issued to Pudelko et al discloses a thick-film potentiometer wherein the wiper track providing the contact area for the moving sweep arm is comprised solely of conductor lines deposited on a substrate in a parallel fashion. The wiper track composed of conductor lines is subject to the small circuit breaks and spikes described earlier. Furthermore, the conductor lines are subject to wear which can lead to a complete failure of the potentiometer at various locations along the wiper track.
Copending application Ser. No. 290,011, filed Dec. 27, 1988, and assigned to Ford Motor Company, teaches a stepped resistor card wherein the conductor lines in the arcuate sweep path have a resistive material deposited thereon. The overprint of resistive material reduces the wear occurring in the sweep path without any substantial effect on the resistance of the variable resistor. However, signal spikes are not eliminated since the gaps between conductor lines are not eliminated.
U.S. Pat. No. 4,500,866, issued to Romann et al, teaches a nonlinear potentiometer including an arcuate resistor pad connected to a plurality of parallel resistor pads by means of a plurality of conductor lines. Such a resistor arrangement is not adaptable to a linear or a well-controlled resistance characteristic because the use of parallel resistors results in an averaging of resistance over the large sweep angles between conductor lines. Furthermore, the specific resistance characteristic that can be obtained in Romann et al is strictly limited by the layout of resistor pads. In addition, the ability to change potentiometer resistance through trimming is limited by the use of parallel resistors pads. Resistor instabilities are caused by extreme trims and the process yield for manufacturing of the potentiometer is low.